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Single-wall carbon nanotubes (SWNTs) were synthesized by the irradiation of a 20-ms CO2 laser pulse (1-kW peak power) onto a graphite−Co/Ni composite target at 25−1200 °C. Characterization of carbonaceous deposits using Raman scattering, scanning electron microscopy, and transmission electron microscopy showed that SWNTs were formed by laser irradiation even at room temperature. At 1100−1200 °C, the SWNT yield significantly increased (> 60%). A high-speed video imaging technique was used to observe the expanding vaporization plume and the emerging carbonaceous materials in an Ar atmosphere. Carbonaceous materials containing SWNTs became visible after ∼3 ms from the initiation of laser irradiation of the target. At 1000−1200 °C, blackbody emission from large carbon clusters and/or particles was observed for more than 1 s after the end of the laser pulse. We suggest that the growth of the SWNTs occurs from a liquidlike carbon−metal particle via supersaturation and segregation. A continuous supply of hot carbon clusters to the particles due to the 20-ms laser pulse and the maintenance of the hot growth zone for SWNTs, performed with the help of a furnace, are thought to play a crucial role in the SWNT formation.

We revealed that the yield of SWNTs formed by Nd:YAG laser ablation depends on the target composition with yields following the order C x Ni y Co y > C x Ni y ≫ C x Co z . The SWNT bundles in the web formed when using the C x Ni y Co y target (web-C x Ni y Co y ) is thicker and longer than those in the web-C x Ni y . The diameters of the SWNTs in the web-C x Ni y Co y were larger and more uniform than those of the SWNTs in the web-C x Ni y . The NiCo particles in the web-C x Ni y Co y and the Ni particles in the web-C x Ni y were nanometer sized and were embedded in the amorphous carbon flakes that were dispersed throughout the weblike deposits. Filmlike deposits were formed when using the C x Co z targets, and nanometer-sized Co particles in these deposits were localized within sub-millimeter-sized areas. Examination of the target surfaces revealed that Ni emits from the C x Ni y target more efficiently than NiCo from the C x Ni y Co y target or Co from the C x Co z target during the laser ablation. On the basis of these results, we provide an explanation of how the yield and structure of SWNTs formed by laser ablation depend on the species of the metal catalysts.

Investigating the formation of single-wall carbon nanotubes (SWNTs) from a target composed of graphite, Ni, and Co, we compared the use of a pulsed CO2 laser (pulse width of 20 ms, laser-power density of 0.1 MW/cm2) with the use of a pulsed Nd:YAG laser (pulse width of 5−7 ns, laser-power density of 4 GW/cm2). When the total irradiation energy was 2 kW/cm2 for the CO2 laser ablation and 2.4 kW/cm2 for the Nd:YAG laser ablation, SWNTs could be formed at 300 K by CO2 laser ablation, whereas the lowest temperature at which they could be formed by Nd:YAG laser ablation was about 1170 K. The lowest pressure allowing SWNT formation was 50 Torr for CO2 laser ablation and 200 Torr for Nd:YAG laser ablation. The structures of carbonaceous deposits and of the target surfaces indicated that CO2 laser ablation resulted in carbon being emitted from the edges of or defects in the graphite particles and most of the Ni and Co being emitted from the target. Pulsed Nd:YAG laser ablation, on the other hand, has been reported to result in graphite melting and mixing with Ni and Co on the target surface, the mixture being expelled from the target surface, and clumps of Ni and Co remaining on the target surface after the laser ablation. We think these differences are due to the different laser-power densities and pulse widths.

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